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ARROW-10266: [CI][macOS] Ensure using Python 3.8 with Homebrew

/usr/local/bin/python3 is a symbolic link to python3.8 but
/usr/local/bin/pip3 is a symbolic link to pip3.9 not pip3.8. We should
use the same Python with pip.

Closes #8426 from kou/homebrew-use-python-3.8

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>